Here's how to use Facebook's tool to check if people you know in Paris are safe

Facebook launched the tool last year to allow people in disaster areas let their friends and loved ones know whether or not they are OK.

To use the tool to mark whether you are safe, or to check on friends in Paris, visit its Paris Terror Attacks page. Users will also get notifications when new friends mark themselves as safe.

A Facebook representative says that the site has activated the tool five times in the past year after natural disasters, but that this is the first time it's had to use it for this kind of situation.

"We are shocked and saddened by the events unfolding in Paris," a representative said. "Communication is critical in these moments both for people there and for their friends and families anxious for news."

Since Facebook activated the tool on Friday, 4.1 million people used it to mark themselves as safe and 360 million people were notified that their friends were safe, the company says.